This invention relates to digitizer tablets and, more particularly, in a digitizer tablet having conductive grids on a substrate, to the improvement to reduce weight and thickness while improving signal strength and shielding characteristics comprising, a shield member adjacent the substrate wherein the shield member is of a metal having a high magnetic permeability.
Digitizer tablets operating according to electro-magnetic principles are well known in the art. A horizontal grid of wires and a vertical grid of wires in the tablet define a square X-Y axis coordinate system. A cursor is then moved over the X-Y axis coordinate system of the tablet to provide operator inputs. Appropriate electronics in the tablet and the cursor allow the position of the cursor on the X-Y axis coordinate system of the tablet to be ascertained. Some times the wires of the grids are the active members to create magnetic fields sensed by the cursor and sometimes the cursor is the active member creating a magnetic field that is sensed in the two grids of wires.
The assignee of this invention favors the latter approach wherein the cursor is the active member. While earlier digitizer tablets had the cursor electrically connected to the tablet with a connecting cable, more recently the cursor is powered with a battery and has no connecting cable to create a so-called "cordless cursor" system.
The assignee of this invention also developed digital tablets wherein the horizontal and vertical grids of wires (which originally were, in fact, wires manually run and soldered to a frame) are formed by runs of electrically conductive ink silk-screened onto a flexible substrate such as Mylar. Such a tablet 10 is depicted in FIG. 1. The tablet 10 comprises a flexible substrate 12 having the grid wires silk-screened thereon according to techniques which are not important to the present invention. Typically, the tablet 10 is formed by laying the substrate 12 on a support member 14 of an insulating material, such as printed circuit board material, and wrapping the edges 16 around the edge of the support member 14. The entire tablet 10 is then mounted inside a protective plastic case which includes the necessary electronics and connective wiring.
While the construction of FIG. 1 reduced the cost of manufacture substantially, particularly in smaller digitizer tablets useful with personal computers and the like, it created certain problems. In particular, as the cursor 18 is moved towards an edge 20 of the working portion of the tablet 10 (i.e. that portion defining the X-Y axis coordinate system), the emitted magnetic field 22 from the cursor 18 passes through the support member 14 and induces undesired currents into the wiring of the edges 16 of the grid system on the substrate 12. This can result in false readings, "jitters", and the like.
To eliminate these problems, the assignee of this invention developed a grid shielding system as depicted in simplified form in FIG. 2. By adding a thin sheet of aluminum as a grid shield 24 under the support member 14, the emitted magnetic field 22 from the cursor 18 passing through the support member 14 is shunted through the aluminum of the grid shield 24 and prevented from reaching the edges 16 of the grid system on the substrate 12.
While the aluminum grid shield 24 of FIG. 2 solved one problem, it created another. When the support member 14 is thin (as only required to support the flexible substrate 12), the aluminum grid shield 24 is positioned too close to the grid wires of the working portion of the substrate 12. Being a good conductive material, the aluminum of the grid shield 24 reduces the effectiveness of the emitted magnetic field 22 from the cursor 18 in inducing sensible signals in to the wires of the grids by the creation of eddy currents in the aluminum of the grid shield 24. To solve this problem, the thickness of the support member is increased to act as a spacer with respect to the aluminum grid shield 24 as depicted in FIG. 3.
Digitizers using a shield plate are known in the art. See, for example, U.S. Pat. Nos. 4,645,869 and 4,956,526. The shielding in the '869 patent attempts to reduce the signal induced in the grid interconnect conductors; but, requires a relatively thick grid assembly. By contrast, the shielding in the system of the '526 patent is for the purpose of blocking disturbing electro-magnetic waves from reaching the grid; and, it is not for the purpose of preventing the signal from reaching the grid interconnect wires on the backside as depicted in FIGS. 2 and 3. The shield plate in these grids still attenuates the signal and the amount of attenuation increases as the space between the grid and the shield plate decreases.
In a corded digitizer tablet wherein power is supplied to the cursor from the tablet and power at the cursor is no problem, the solution of FIG. 3 in various embodiments for different tablet constructions worked well for its intended purpose. With the introduction of the cordless cursors, however, a new problem developed. The cordless cursors are powered by a small lithium hearing aid type of battery. Power and battery drain, therefore, become a significant problem. Signal strength of the magnetic field from the cordless cursor is kept to a minimum in order to minimize the power consumption. With this low-power signal, the construction of FIG. 3 does not work well. While the aluminum grid shield spaced from the grid wires blocks most of the magnetic field of the cursor from reaching the backside wires, the eddy currents essentially still buck the coordinate signal being developed and a significant portion of the signal is still lost. To make it work, the output of the transducer in the cursor (and corresponding battery current) must be high enough to overcome the attenuation. As a result, power consumption is much larger than desired for cordless cursor operation and battery life. Also, in the new, smaller cordless digitizer tablets being developed for pen-driven computing systems, the thickness of the aluminum and the spacing support member required for proper operation make the resultant tablet much thicker and heavier than desirable.
Wherefore, it is an object of the present invention to provide a way in which the thickness and weight of a cordless digitizer tablet can be reduced without impairing its operability.
It is another object of the present invention to provide a cordless digitizer tablet having a cursor with minimal battery current drain requirements.
It is still another object of the present invention to provide a way of shielding the grids of a cordless digitizer tablet without diminishing the strength of the signal of interest.
It is yet another object of the present invention to provide a way of shielding the grids of a cordless digitizer tablet in a manner which also increases the strength of the signal of interest.
Other objects and benefits of the invention will become apparent from the detailed description which follows hereinafter when taken in conjunction with the drawing figures which accompany it.